As is well known in the surge arrester art, and as is described, for example, in Adveenko et al U.S. Pat. No. 4,298,900, issued Nov. 3, 1981, surge arresters commonly have comprised a non-linear voltage-dependent resistor arrangement contained within the bore of an externally shedded glazed porcelain insulator housing. The non-linear resistor arrangement commonly has comprised a series arrangement of varistor elements formed principally of silicon carbide or zinc oxide, for example, and, as disclosed in Matsuoka et al U.S. Pat. No. 3,805,114, issued Apr. 16, 1974, the varistor elements can be in series with one or more discharging or spark gaps. In the event of a voltage surge on a line connected to ground (earth potential) by way of the surge arrester, the spark gaps are adapted to spark over and to put a transient earth fault on the line for the duration of the surge, the series varistor elements adopting a temporary low resistance condition such as to limit the power follow current which can flow through the arrester to a level which can be cleared by the spark gaps.
The commonly utilized porcelain housing has placed severe and serious constraints upon the manufacture of surge arresters. It has been conventional to vacuum dry the assembly of the non-linear resistor arrangement with the porcelain housing and to fill the arrester with an inert gas prior to final sealing. It is also conventional, particularly in the case of larger sized, station class arresters but also in the case of smaller distribution type arresters, to provide a pressure relief diaphragm at one or both ends of the porcelain insulator housing, as disclosed for example in U.S. Pat. No. 4,298,900 above-mentioned, with such diaphragm(s) being adapted to rupture in the event of an over-pressure occurring in the gas filling of the arrester as a result of the expansion of this gas when the arrester operates to divert a current surge to ground. The purpose of these provisions has been to protect the porcelain housing against explosive shattering which otherwise can occur with corresponding risk to life and property; the bush fires which in recent years have ravaged vast areas of Australia are believed to have been caused, in some instances at least, by explosive shattering of porcelain arrester housings and the resultant spewing out of high temperature fragments over an area of several hundred square yards.
The cost of the porcelain housing itself contributes very significantly to the cost of a conventional surge arrester provided with such a housing, and the cost of the treatments and arrangements which have been considered necessary to minimize or isolate the danger it presents by virtue of its risk of explosive shattering have caused the overall cost of a surge arrester to be relatively high. Given these disadvantages, there have been various previous attempts to construct a surge arrester, suitable at least for electrical power distribution applications, which does not use a porcelain insulator housing, but to our knowledge and belief no such surge arrester has been commercially available.
In British Pat. No. 867901 (Westinghouse), issued May 10, 1961, the above and other disadvantages of porcelain housings in surge arresters are discussed, and it is proposed instead to use a plastic molded housing encasing the spark gaps and varistor blocks. However, the patent goes on to discuss problems of deterioration of the plastic housing by exposure to the ultraviolet radiation in sunlight, and proposes as one solution to these to shroud the polyethylene housing within an outer porcelain housing. This patent thus seeks to solve the problems of porcelain housings by means of a solution which itself gives rise to such problems as to invite the addition of a porcelain housing to alleviate them. The problems of porcelain arrester housings are also discussed in Westrom U.S. Pat. No. 3,727,108, issued Apr. 10, 1973, which proposes instead to encapsulate the spark gaps and varistor blocks of the arrester within a housing formed of elastomeric material. In West German Pat. No. 1,638,120 (Siemens AG), there is disclosed a surge arrester in which the arrester housing is of molded synthetic resin material. Despite these prior proposals, surge arresters to the present day commonly comprise porcelain housings, notwithstanding the problems associated therewith.
Yet another proposal is contained in British Pat. No. 2,073,965, assigned to the assignees of the present invention and issued May 2, 1984, and comprises a surge arrester formed as a stack of varistor blocks capped at each end by means of an appropriate termination and with a shedded sleeve of heat-shrink material encasing the assembly, the heat-shrink material being a high voltage electrical insulator capable of withstanding the electric potential of a lightning strike, for example, without dielectric breakdown. Such a surge arrester promised to provide an elegant and advantageous solution to the porcelain insulator problem and with other advantages of ease of fabrication and cost benefit. However, while the design of the proposed surge arrester was elegantly simple and is theoretically viable, experience has shown that the proposed arrester did not lend itself to volume manufacture.
A similar surge arrester to that proposed in British Pat. No. 2,073,965 abovementioned is described in Russian Patent Specification No. 853,728, and comprises a stack of varistor blocks interleaved with metal contact plates and encased within a heat-shrink material housing. The surge arrester described in this Russian patent is not considered to be an efficacious design on account of the inevitability that the shrinking down of the heat-shrink material housing onto the assembled varistor blocks and metal contact plates would entrap air, thereby leading to risk of local ionization and flashover failure of the arrester. The disclosure of this Russian patent thus does not provide a solution to the problems of prior art surge arresters.